This invention relates generally to a double resist layer lift-off technique for high energy radiation lithography and more particularly to highly sensitive resist layers having different solvent development systems.
The formation of positive resist masks from layers of radiation degradable polymers is described, for example, by Haller and Hatzakis in U.S. Pat. No. 3,535,137. In this process a radiation degradable polymer layer is coated on a substrate and is subjected to patternwise exposure to high energy radiation such as, for example, x-rays, nuclear radiation, and electrons. The irradiated regions of the polymer suffer a decrease in molecular weight and thereby become more soluble. A developer is then used to preferentially remove the irradiated portions of the layer. The substrate can then be subjected to an additive or subtractive process such as metallization or etching with the remaining portions of the resist layer protecting the substrate from the processing.
In the formation of integrated circuits or exposure masks, one process which is employed is a lift-off process in which a patterned relief layer of resist is first formed on a substrate. A layer of material, such as a metal for integrated circuit conductor lines or an opaque masking material for mask fabrication is coated over the resist layer and the exposed portions of the substrate. The resist layer is then stripped off and takes with it the overlying material to leave only the pattern of material in direct contact with the substrate. Such a process is described, for example, by M. Hatzakis, Electron Resists for Micro Circuit and Mask Production, Journal of the Electro Chemical Society, Volume 116, No. 7, pages 1033 to 1037, July 1969 and by M. Hatzakis and A. Broers, Record of the Eleventh Symposium on Electron, Ion and Laser Beam Technology, pages 337 to 344.
The described process takes advantage of the natural undercutting of the resist during high energy exposure such that the developed resist pattern is wider at the bottom then the top. This profile aids in forming a discontinuity between the portions of material which are on the substrate surface and the portions which cover the resist. This discontinuity is needed in order to permit the resist stripping solution to attack the unexposed resist and remove it along with the overlying material.
The resist layer thickness required for a metal lift-off process, for example, must be in the ratio of a minimum of about 1.5/1 resist thickness to metal thickness to avoid bridging of metal between the portion on the substrate and the portion covering the resist. Therefore, the loss of unexposed resist during development must be limited. This can be done by increasing exposure times to create a greater molecular weight differential between the exposed and unexposed resist. However, this has the effect of slowing down the exposure process.
Another factor involved in the use of high energy radiation to exposed resist layers is the fact that there are certain advantages to the use of, for example, beams of increasing energy, for example, 10 to 50 kilovolts (KV). This has the advantage of decreasing writing times for a resist because higher amps/per square centimeter can be produced by the electron gun and the higher energy beams also provide more back scatter electron signal for better registration and detection. However, it has been found that for any given resist thickness employed, the higher energy beams produce less electron scattering and undercutting of the resist such that excessive exposure times would be required to obtain the desired undercut profile by exposure alone.
A method of enhancing metallurgical lift-off is disclosed in commonly assigned U.S. Application Ser. No. 426,403 now U.S. Pat. No. 3,934,057 (427/43). The application teaches the use of 2 or more layers of resist, each resist having a different dissolution rates. The layer can be of the same or different polymeric material. The differing dissolution rate can be due to differing molecular weight polymers or of differing tacticity. The layered materials used therein can be dissolved by the same solvent. This method does not provide sufficient improvement in resist sensitivity to allow lift-off below exposures of 2 .times. 10.sup.-.sup.5 coulombs/cm.sup.2. At exposures lower than 2 .times. 10.sup.-.sup.5 coulombs/cm.sup.2 the required development time is so long that the undercut feature of the resist combination can not be maintained. Although the dissolution rate of the two or more resist films in this process is different, the top resist film has a finite development rate which causes the opening on the top film to widen and destroy the undercut if the total development time is long.
One of the limitations of a high speed electron beam fabrication system is resist speed, especially for lift-off metallization where high exposure is required to provide some undercut in the developed resist profile. Until the discovery of the present invention no known assist system was available which could operate for lift-off at 5 .times. 10.sup.-.sup.6 coul/cm.sup.2 or even 10.sup.-.sup.5 coulombs/cm.sup.2.